Fusarium oxysporum was isolated most frequently, followed by F. moniliforme, and F. solani from infected asparagus plants grown in the field. In pathogenicity tests both with seedlings and plantlets, F. moniliforme showed slightly higher virulence than Fusarium oxysporum did in general. Fusarium moniliforme showed more consistent virulence on both seedlings and plantlets than F. oxysporum did. Fusarium oxysporum showed slightly higher virulence on plantlets than on seedlings. Fusarium solani showed very weak or no sign of virulence on seedlings and plantlets, respectively, in both tests. In protection tests with plantlets, most protection of asparagus against virulent fusarial infections occurred when challenge isolates were inoculated five or seven days after inoculation of protective fusarial species. Avirulent F. oxysporum was a more effective protective agent against infection of F. moniliforme than it was against F. oxysporum. Fusarium solani was more effective against infection of F. oxysporum than it was against F. moniliforme. Virulent fusarial species showed significant increase in conidial populations on asparagus plantlet root and stem segments, and showed higher root and stem rot ratings than avirulent fusarial species did. Avirulent fusarial species showed minimal increase in conidial populations on both root and stem segments, and showed low to very low root and stem rot ratings. All fusarial species infected asparagus plantlets through primary and lateral root tips, natural wounds, and between the walls of the epidermal cells directly. Some penetration was appressorium-like and direct. It was assumed that the meristematic region could act as a major infection site. Virulent fusarial species were growing faster and more abundantly inside and outside of the plantlet epidermal areas than were avirulent fusarial species. Fusarium solani was the slowest growing species. Within a short period, virulent fusarial species caused cortical rots. However, over extended periods, they invaded tracheary elements eventually, and caused extensive damages. Avirulent F. oxysporum accumulated heavily on and around the epidermal areas even if it invaded part of cortical cells inside the epidermal regions. Fusarium solani caused proliferation of lateral roots and increased the surface area of primary and secondary roots
